1. Field of the Invention
The present invention relates to the field of laser material processing methods and systems, and specifically, to scanned laser processing methods and systems for the processing of semiconductor wafers, electronic substrates, and workpieces to be laser micromachined.
2. Background Art
Conventionally, round spots have been used for a majority of precision-scanned, laser-processing applications. Many laser sources such as Nd:YAG lasers produce round Gaussian beams, which, when imaged through conventional spherical optics, produce round spots. These spots are scanned across target sites to process material, and the resulting laser-material interaction removes or otherwise alters the targeted material. In many laser-processing applications, system throughput is limited by the average power, the substrates"" fluence damage threshold, and, in pulsed systems, the laser q-rate and the laser pulse characteristics.
Exemplary micromachining operations include link blowing of redundant memory circuits, laser trimming, and circuit fabrication. For processing applications such as blowing sub-micron width fuses on a memory device, efficient coupling of energy to a narrow fuse with minimum lateral and substrate damage is desirable. Large round spots may cause undesirable adjacent link damage as shown in FIG. 19a. While smaller spots allow finer pitched fuses to be processed, the potential for substrate damage can increase with the higher fluence of a decreasing spot size as shown in FIG. 19b. When micromachining a line made from a sequence of small laser spots, the spot overlap or so-called xe2x80x9cbite sizexe2x80x9d and q-rate are two of the process characteristics that determine maximum scan velocity. Laser trimming applications requiring a wide kerf width may require multiple passes with a small spot when the peak fluence of a larger spot is inadequate. In the field of a lead frame fabrication, wherein a fine pitch lead on a large lead count device is machined, a rotating elongated spot is used as described in U.S. Pat. No. 5,632,083.
Out-of-round spots are often considered as system defects that limit process quality. Much effort has been expended in the field of laser optics to improve beam quality, to circularize beams from diode lasers, and to design and implement highly corrected optics for diffraction-limited systems. Vector diffraction effects used for beam-shape compensation are described in U.S. Pat. No. 4,397,527.
Many techniques are known for beam shaping and spot shaping. One method is a phase plate used with a round beam to modify the spot shape for processing memory fuses as shown in the upper portion of FIG. 20 and as taught by Cordingley in U.S. Pat. No. 5,300,756. The primary effect using this simple type of phase plate is to create a top-hat distribution profile as shown in the lower portion of FIG. 20. However, techniques for creating an oblong spot are also described.
Use of an anamorphic spot with dithering for shaping a laser beam intensity profile is described in U.S. Pat. No. 6,341,029. The anamorphic spot allows sharper line edges to be formed with a narrowed spot width, while an increased spot length maintains desired total power without exceeding process limits on integrated power per unit substrate area.
Veldkamp in U.S. Pat. No. 4,410,237 describes a diffraction grating and prism method for transforming a round Gaussian beam to elongated flat-top profiles.
Dickey in U.S. Pat. No. 5,864,430 describes a phase-based method for transforming a round Gaussian beam to a flat-top, square, or rectangular-shaped spot.
Yet another technique is creating an array of spots such as disclosed by James in U.S. Pat. No. 5,463,200.
Another well known technique is the imaged aperture mask.
Apodization is yet another simple technique to modify the beam shape and thereby spot shape.
Published U.S. patent application in the name of Baird et al., US 2002/0005396 A1, discloses a UV laser system wherein an optics module is provided to enhance shape quality of laser beams.
Sun et al. in U.S. Pat. No. 5,265,114 describe a method and system for selectively laser processing a target structure of one or more materials of a multi-material, multi-layer device.
Sun et al. in U.S. Pat. No. 6,057,180 describe a method for severing electrically conductive links with ultraviolet laser output.
It is an object of at least one embodiment of this invention to sequentially and relatively position a modified laser beam into at least one spot having an adjustable aspect ratio for laser material processing of one or more targets within a field in a variety of micromachining applications.
It is an object of at least one embodiment of the present invention to correct system-induced, spot non-uniformity to create precise round or elongated spots such as elliptical spots.
In carrying out the above objects and other objects of the present invention, a high-speed, precision, laser-based method for processing material of at least one target within a field is disclosed. The method includes generating a laser beam along a propagation path, and controllably modifying the laser beam to obtain a modified laser beam. The method also includes sequentially and relatively positioning the modified laser beam into at least one spot at each target within the field to process the material of each target wherein the at least one spot has a set of desired spatial characteristics including an adjustable aspect ratio which are obtained by the step of controllably modifying.
The at least one spot may have a pair of axes and the modified laser beam may be focused to a substantially common point in both axes.
The adjustable aspect ratio may be greater than 0.1 but less than 10.
Multiple targets may be within the field.
The step of sequentially and relatively positioning may include the step of vector scanning the modified laser beam.
The step of controllably modifying the laser beam may be repeated during the step of sequentially and relatively positioning the modified laser beam so that the at least one spot has at least a second set of desired spatial characteristics.
The set of desired spatial characteristics may be determined based on at least one target characteristic of each target.
The set of desired spatial characteristics may also be determined based on at least one target material property of each target, or based on at least one process variable.
The set of desired spatial characteristics may further be determined based on at least one desired laser material processing characteristic.
The step of generating may include the step of shaping the laser beam to change the aspect ratio and to obtain a modified beam having a first elongated irradiance pattern with a first orientation wherein the modified beam is delivered and focused into the at least one spot.
The step of generating may also include the step of further shaping the modified beam to obtain a laser beam having a second elongated irradiance pattern with a second orientation.
The step of controllably modifying may control absolute orientation of the first and second orientations based on target orientation.
The step of shaping may further include the step of controlling relative orientation of the first and second orientations.
The step of sequentially and relatively positioning may deliver and focus the beam into a plurality of spots extending along a laser processing path wherein the aspect ratio and orientation of each of the spots is based on predetermined dimensions of each target and target orientation.
The laser processing path may be a curvilinear path.
Each of the spots may be an elongated spot having a major axis and wherein the major axis of at least one of the spots is aligned with the laser processing path.
Each of the spots may also be an elongated spot having a major axis and wherein the major axis of at least one of the spots is transverse the laser processing path.
An aspect ratio and orientation of the at least one spot may be controlled based on predetermined dimensions of each target and target orientation.
The step of generating may include the step of filtering the laser beam to obtain an initially modified spot shape.
The step of generating may also include the step of filtering the laser beam to obtain an initially modified spot irradiance profile.
The processing may be micromachining and may include semiconductor link removal, laser trimming, laser drilling or laser etching.
The step of sequentially and relatively positioning may deliver and focus the modified beam into a spot which is scanned along a laser processing path wherein an aspect ratio and orientation of the spot is based on predetermined dimensions of each target and target orientation.
The modified beam may have an irradiance pattern of an elliptical Gaussian beam.
The modified beam may have an irradiance pattern of a top hat in one direction and Gaussian in a direction orthogonal to the one direction.
Further in carrying out the above objects and other objects of the present invention, a high-speed, precision system for processing material of at least one target having predetermined dimensions and a characteristic within a field is disclosed. The system includes a laser source for generating a laser beam along a propagation path having an irradiance pattern with an aspect ratio and an orientation in a plane substantially perpendicular to the propagation path. A controller generates control signals including orientation control signals based on the characteristic. A first subsystem is disposed in the propagation path for shaping the laser beam based on the predetermined dimensions to change the aspect ratio and obtain a modified beam. A second subsystem controllably changes the orientation of the irradiance pattern based on the orientation control signals. A beam delivery and focusing subsystem sequentially positions and focuses the modified beam into at least one spot on each target to process the material of each target.
The first subsystem may include a first anamorphic optical device for shaping the laser beam to obtain an initially modified laser beam having a first elongated irradiance pattern with a first orientation.
The first subsystem may also include a second anamorphic optical device for further shaping the initially modified laser beam to obtain the modified beam having a second elongated irradiance pattern with a second orientation.
The second subsystem may include at least one actuator for moving the anamorphic optical devices to control absolute orientation of the devices in response to the orientation control signals.
One actuator may move one of the anamorphic optical devices to control relative orientation of the devices in response to the control signals.
The beam delivery and focusing subsystem may sequentially position and focus the modified beam to a plurality of spots extending along a laser processing path wherein the aspect ratio and the orientation of each of the spots is based on the predetermined dimensions and target orientation.
The laser processing path may be a curvilinear path.
Each of the spots may be an elongated spot having a major axis wherein the major axis of at least one of the spots is aligned with the laser processing path.
Each of the spots may further be an elongated spot having a major axis wherein the major axis of at least one of the spots is transverse the laser processing path.
The first subsystem may include an ellipse generator wherein the irradiance pattern of the modified beam is an elliptical irradiance pattern and the at least one spot is at least one elliptical spot.
The second subsystem may include a beam rotator for rotating the modified beam.
An aspect ratio and orientation of the at least one spot may be controlled based on the predetermined dimensions and target orientation.
The processing may be micromachining and may include semiconductor link removal, laser trimming, laser drilling or laser etching.
The system may compensate for system errors that would result in an out-of-round or an out-of-ellipse condition.
The first subsystem may include a fixed expander/rotator based on an in-line or compensated offset optical path.
The first subsystem may include a plurality of non-offset prism pairs.
The first subsystem may include a multi-element, sphero-cylindrical expander.
The system may further include a filter for filtering the laser beam to obtain an initially modified spot shape.
The second subsystem may include an adaptive optical element.
The first subsystem may include an adaptive optical element.
The system may further include a filter for filtering the laser beam to obtain an initially modified spot irradiance profile.
The beam delivery and focusing subsystem may sequentially position and focus the modified beam into a spot that is scanned along a laser processing path wherein the aspect ratio and orientation of the spot is based on the predetermined dimension and target orientation.
The irradiance pattern may be an elliptical Gaussian beam.
The irradiance pattern may be a top hat in one direction and Gaussian in a direction orthogonal to the one direction.
Further in carrying out the above objects and other objects of the present invention, a high-speed, precision, laser-based method for processing material of at least one target within a field is disclosed. The method includes the steps of: a) generating a laser beam along a propagation path; b) controllably modifying the laser beam to obtain a modified laser beam; c) relatively positioning the modified laser beam into at least one spot at a target within the field to process the material of the target wherein the at least one spot has a set of desired spatial characteristics including an adjustable aspect ratio which are obtained by step b); and d) repeating steps a) through c) for each target until the material of all targets within the field are processed.
The method may further includes the step of: e) evaluating at least one of process, material and target characteristics to obtain data wherein step b) is based on the data, and wherein step d) repeats steps a) through c) and step e) for each target until the material of all targets within the field are processed.
The above objects and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.